PAICS ubiquitination recruits UBAP2 to trigger phase separation for purinosome assembly

Purinosomes serve as metabolons to enhance de novo purine synthesis (DNPS) efficiency through compartmentalizing DNPS enzymes during stressed conditions. However, the mechanism underpinning purinosome assembly and its pathophysiological functions remains elusive. Here, we show that K6-polyubiquitination of the DNPS enzyme phosphoribosylaminoimidazole carboxylase and phosphoribosylaminoimidazolesuccinocarboxamide synthetase (PAICS) by cullin-5/ankyrin repeat and SOCS box containing 11 (Cul5/ASB11)-based ubiquitin ligase plays a driving role in purinosome assembly. Upon several purinosome-inducing cues, ASB11 is upregulated by relieving the H3K9me3/HP1α-mediated transcriptional silencing, thus stimulating PAICS polyubiquitination. The polyubiquitinated PAICS recruits ubiquitin-associated protein 2 (UBAP2), a ubiquitin-binding protein with multiple stretches of intrinsically disordered regions, thereby inducing phase separation to trigger purinosome assembly for enhancing DNPS pathway flux. In human melanoma, ASB11 is highly expressed to facilitate a constitutive purinosome formation to which melanoma cells are addicted for supporting their proliferation, viability, and tumorigenesis in a xenograft model. Our study identifies a driving mechanism for purinosome assembly in response to cellular stresses and uncovers the impact of purinosome formation on human malignancies.

Alternative Splicing of RAD6B and Not RAD6A is Selectively Increased in Melanoma: Identification and Functional Characterization

Rad6B, a principal component of the translesion synthesis pathway, and activator of canonical Wnt signaling, plays an essential role in cutaneous melanoma development and progression. As Rad6 is encoded by two genes, namely, UBE2A (RAD6A) and UBE2B (RAD6B), in humans, we compared their expressions in melanomas and normal melanocytes. While both genes are weakly expressed in normal melanocytes, Rad6B is more robustly expressed in melanoma lines and patient-derived metastatic melanomas than RAD6A. The characterization of RAD6B transcripts revealed coexpression of various splice variants representing truncated or modified functional versions of wild-type RAD6B in melanomas, but not in normal melanocytes. Notably, two RAD6B isoforms with intact catalytic domains, RAD6BΔexon4 and RAD6Bintron5ins, were identified. We confirmed that RAD6BΔexon4 and RAD6Bintron5ins variants are expressed as 14 and 15 kDa proteins, respectively, with functional in vivo ubiquitin conjugating activity. Whole exome sequence analysis of 30 patient-derived melanomas showed RAD6B variants coexpressed with wild-type RAD6B in all samples analyzed, and RAD6Bintron5ins variants were found in half the cases. These variants constitute the majority of the RAD6B transcriptome in contrast to RAD6A, which was predominantly wild-type. The expression of functional RAD6B variants only in melanomas reveals RAD6B's molecular heterogeneity and its association with melanoma pathogenesis.

Melatonin receptors and their regulation: biochemical and structural mechanisms

There is growing evidence demonstrating the complexity of melatonin's role in modulating a diverse number of physiological processes. This complexity could be attributed to the fact that melatonin receptors belong to two distinct classes of proteins, that is, the G-protein coupled receptor superfamily (MT(1), MT(2)) and the quinone reductase enzyme family (MT(3)) which makes them unique at the molecular level. Also, within the G-protein coupled receptor family of proteins, the MT(1) and MT(2) receptors can couple to multiple and distinct signal transduction cascades whose activation can lead to unique cellular responses. Also, throughout the 24-hour cycle, the receptors' sensitivity to specific cues fluctuates and this sensitivity can be modulated in a homologous fashion, that is, by melatonin itself, and in a heterologous manner, that is, by other cues including the photoperiod or estrogen. This sensitivity of response may reflect changes in melatonin receptor density that also occurs throughout the 24-hour light/dark cycle but out of phase with circulating melatonin levels. The mechanisms that underlie the changes in melatonin receptor density and function are still not well-understood, but data is beginning to show that transcriptional events and G-protein uncoupling may be involved. Even though this area of research is still in its infancy, great strides are being made everyday in elucidating the mechanisms that underlie melatonin receptor function and regulation. The focus of this review is to highlight some of these discoveries in an attempt to reveal the uniqueness of the melatonin receptor family while at the same time provide thought-provoking ideas to further advance this area of research. Thus, a brief overview of each of the mammalian melatonin receptor subtypes and the signal transduction cascades to which they couple will be discussed with a greater emphasis placed on the mechanisms underlying their regulation and the domains within the receptors essential for proper signaling.

Solid-phase synthesis of thalidomide and its analogues

A novel solid-phase synthesis of thalidomide and its metabolites and analogues is described. The synthetic strategy involves the coupling of hydroxymethyl polystyrene with phthalic anhydride to form the resin-linked acid. The acid is then reacted with primary amines followed by acid or base treatment to form thalidomide and its analogues with either open or closed phthalimide rings. Most of the analogues are synthesized with high yields (40.3-98.1% in three steps) and purities (92.3-98.9%).

Molecular structure of Escherichia coli PurT-encoded glycinamide ribonucleotide transformylase

In Escherichia coli, the PurT-encoded glycinamide ribonucleotide transformylase, or PurT transformylase, catalyzes an alternative formylation of glycinamide ribonucleotide (GAR) in the de novo pathway for purine biosynthesis. On the basis of amino acid sequence analyses, it is known that the PurT transformylase belongs to the ATP-grasp superfamily of proteins. The common theme among members of this superfamily is a catalytic reaction mechanism that requires ATP and proceeds through an acyl phosphate intermediate. All of the enzymes belonging to the ATP-grasp superfamily are composed of three structural motifs, termed the A-, B-, and C-domains, and in each case, the ATP is wedged between the B- and C-domains. Here we describe two high-resolution X-ray crystallographic structures of PurT transformylase from E. coli: one form complexed with the nonhydrolyzable ATP analogue AMPPNP and the second with bound AMPPNP and GAR. The latter structure is of special significance because it represents the first ternary complex to be determined for a member of the ATP-grasp superfamily involved in purine biosynthesis and as such provides new information about the active site region involved in ribonucleotide binding. Specifically in PurT transformylase, the GAR substrate is anchored to the protein via Glu 82, Asp 286, Lys 355, Arg 362, and Arg 363. Key amino acid side chains involved in binding the AMPPNP to the enzyme include Arg 114, Lys 155, Glu 195, Glu 203, and Glu 267. Strikingly, the amino group of GAR that is formylated during the reaction lies at 2.8 A from one of the gamma-phosphoryl oxygens of the AMPPNP.